Noble metals (or precious metals) are not only in demand for jewelry but also in demand industrially due to electrical and chemical properties thereof, for example, an electronic apparatus, including a printed wiring board. Noble metals are also widely used for dental materials, medical materials, various catalysts, and others. In particular, nowadays the prices of the noble metals are rising against the backgrounds of the expansion of the overseas market and the speculation purposes due to economic anxiety. Thus, for the purpose of cutting down production costs, it is urgently necessary to reduce the amount of noble metals to be used or to recycle noble metals.
As a method for recovering a noble metal from a wastewater (or a waste fluid) containing a low concentration of the noble metal, use of an ion exchange resin or an activated carbon is known. Unfortunately, the ion exchange resin or the activated carbon has a low capacity to absorb a noble metal in some wastewaters that have a high acid concentration, a high alkali concentration, a high salt concentration, a high impurity concentration, or the like. Thus the use of the ion exchange resin or the activated carbon is industrially unsatisfactory. Another known method includes use of a resin having a special functional group added thereto for adsorbing a noble metal selectively. Unfortunately, not only does the resin hardly adsorb a low concentration of a noble metal ion, but the production cost of the resin is high. Thus repeated use of the resin requires alternate adsorption and desorption. Moreover, the decrease in adsorption rate, desorption rate and purity due to repeated use is also difficult to inhibit sufficiently. In a case where the resin is burned in recovering noble metals undesorbed finally, there is a problem that harmful components are released to the environment. Further, this method needs a high recovery cost and is industrially unsatisfactory.
Moreover, a method using an activated carbon for recovering a noble metal is also known. For example, Japanese Patent Application Laid-Open Publication No. 2009-24200 (JP-2009-24200A, Patent Document 1) reports a method for separating and recovering a noble metal; the method comprises: dissolving a metal component of a waste electronic part in an aqueous separation solution containing a cyan ion, adding sodium hypochlorite to the resulting solution to maintain an oxidation-reduction potential of not less than +350 mV (silver/silver chloride electrode) and further adding a mineral acid to the solution to adjust the pH to 6 to 8, then throwing an activated carbon into the solution while stirring to adsorb a noble metal on the activated carbon, separating the activated carbon by filtration, and recovering the noble metal from the activated carbon separated.
Unfortunately, this method requires complicated steps, and in addition, there is a variation in the concentration of the noble metal in the solution every batch. Thus in order to recover the noble metal efficiently, it is necessary to determine the amount of the activated carbon required for adsorbing a noble metal beforehand by a spot test. This method has a low productivity and is unfavorable in view of industry.
Japanese Patent Application Laid-Open Publication No. 2001-303148 (JP-2001-303148A, Patent Document 2) reports a method for treating a detergent drain containing palladium; the method comprises: adding a coagulant to a detergent drain containing a palladium catalyst added in an electroless plating step to precipitate a palladium-containing slurry, then dissolving the palladium-containing slurry in a mineral acid, and passing the resulting dissolved solution through a capture column of an activated carbon loaded with a reducing agent to adsorb palladium metal on the column. This document discloses that a hydroquinone derivative is effectively used as the reducing agent.
Although this method can relatively easily recover a noble metal, the method costs a great deal because of the hydroquinone derivative used as the reducing agent. In addition, since the hydroquinone derivative plugs pores of the activated carbon due to adsorption, the adsorption amount of the noble metal is also insufficient.